"Optimizing Fluid Allocation in Shell and Tube Heat Exchangers"

 

Fluid Allocation in Shell and Tube Heat Exchangers:           A Simplified Guide

 

 

 

Fluid allocation in shell and tube heat exchangers is a crucial and critical decision of the design process in general , impacting heat transfer efficiency, maintenance requirements, and overall cost-effectiveness of given exchanger. Let's explore the key aspect to consider for determining which fluid should be placed or allocated to the tube-side and which to the shell-side .

 

 

1. Fouling Factor: Fouling is a deposition of unwanted substance as scale, sediment, biological material either by bio growth, chemical reactions or corrosion on the inside surface of heat exchanger and inside and outside of tubes of exchanger. It results negatively as insulates, lowers heat transfer, burns more energy, increases drop in pressure & lowers flow rate thus impacting performance and efficiencies. 

• Fluids prone to fouling should be allocated to the tube-side.
• The tubes are easier to clean, thus prolonging the exchanger's lifespan.
• Higher velocities in the tubes can help reduce fouling.
  

  
  

                                                 Tube Fouling                                               Biological Fouling 

2. Pressure:

• Higher pressure fluids should always be placed in the tube-side.
• Placing high-pressure fluids in the tubes allows for a reduction in shell thickness, minimizing material costs as tubes are of small diameter compared to shell diameter requires less wall thickness than shell to withstand same pressure.

 

3. Corrosion Rate: Corrosion is effect on metal by chemical reaction with its environment leading rust, thinning of wall, pitting thus weakening heat exchanger to prone for leak or failures.

• Fluids with higher corrosion rates should be allocated to the tube-side.
• Fewer corrosion-resistant alloys are required when corrosive fluids are placed on the tube-side, reducing costs.
  
                                        Corroded Tubes and tube sheet               Corroded dish end cover

 

4. Viscosity: It is fluid's resistance to freely flow, thick /high viscous flows slowly and thin /less viscous fluids quickly flows. More energy required to pump through heat exchanger for highly viscous fluid, still result into high pressure drop most of the times.

• Fluids with higher viscosity are typically allocated to the shell-side.
• Shell-side allocation maximizes heat transfer rates, especially with viscous fluids.
  

  
  
  
                                          High Viscous Fluid                                     Low Viscous Fluid

 

 

5. Phase Change:

• Fluids undergoing phase change are assigned to the shell-side.
• In steam heated vaporizers/reboilers, condensing steam is placed in the tube-side.
  
  
  
  
  
  Reboiler :- Phase change from steam to condensate & liquid feed to vapour
• 
  
  
  
  

         Water cooled Condenser :- Phase change of Vapor refrigerant to liquid refrigerant in HVAC 

It's important to note that each situation may have unique considerations, and conflicting factors may arise. In such cases, a careful evaluation of the specific requirements is necessary. Every customer may have there own priority for various factors such as initial cost, maintenance, service, brake down cost, energy efficiency, locational advantages & disadvantages and resources.

  

Table A :- Allocation of Fluids on the Shell and Tube Sides

Sr.	Fluid Attributes	Shell side Fluid placement	Tube side Fluid placement	Remark
1	Fluid with Elevated Pressure	No	Yes
2	More Corrosive Fluid	No	Yes
3	Fluid Prone to Fouling	No	Yes
4	Hazardous Fluid	No	Yes
5	Viscous Fluid	Yes	No
6	Fluid with Reduced Flow Rate	Yes	No
7	Fluid with Diminished Heat Transfer Coefficient	Yes	No
8	Fluid that solidifies	No	Yes	Special case
10	Fluid undergoes phase change	Yes	No
		Condensing Vapours	Condensing steam - Reboiler	Special case

 

Additional Considerations:

 

• Maintenance: Tube-side allocation simplifies cleaning procedures and reducing maintenance efforts.
• Thermal Performance: Proper fluid allocation optimizes heat transfer efficiency to enhancing overall performance.
• Design Flexibility: Shell-side allocation provides more options for dealing with high pressure drops or low heat transfer coefficients .

Ultimately, fluid allocation begins during the design phase and requires thoughtful deliberation to ensure the cost, efficiency, reliability, and longevity of the every heat exchanger.

 

This simplified guide aims to provide clarity on the various complex decision-making process involved in fluid allocation for shell and tube heat exchangers. By considering factors such as fouling, pressure, corrosion, viscosity, and phase change, engineers can make informed decisions to optimize heat exchanger performance and minimize maintenance costs.